The PCI specification adopted by Apple in its new machines conforms to PCI Specification 2.3, which does not support 5V PCI cards, as it introduces a new card architecture.

MPEG and DVD encoder-solutions company Wired Inc's president Mike Evangelist warns that "5V cards will not even fit into the sockets".

Former Apple staffer Evangelist said: "Numerous professional Mac users who already have one of our boards will never be able to use it in a newer Mac – and there is no way to retrofit an existing board."

Evangelist's company has until recently been producing PCI boards based on the 5V architecture.

Evangelist explained that his company has redesigned its encoder boards to function with the new Macs, but believes that some pro users may be unaware of the potential impact of the new architecture:

Evangelist isn't unhappy with Apple's decision to introduce a new era of PCI-card performance. "I am one of Apple's biggest supporters, and I love the new G5," he said.

"But I feel it's important for professional users to know that their investment in PCI cards may not move forward with them."

The majority of professional Mac users looking to upgrade to the G5 will be unaffected, Apple sources said: "Most current PCI cards will work just fine in the Power Mac G5."

The majority of installed PCI cards today run at 3.3V, and these are supported in the new G5s.

More information regarding Apple's new PCI technology and G5 Power Mac is available here.